Method and apparatus to control a power converter having a low loop bandwidth

ABSTRACT

An example controller includes a feedback sensor circuit that receives a feedback signal representative of an output of a power converter. A feedback sampling signal generator is coupled to generate a feedback sampling signal. The feedback sensor circuit samples the feedback signal in response to the feedback sampling signal. A state machine controls switching of a switch of a power converter circuit according to one of a plurality of operating condition states in response to the feedback sensor circuit. Each of the plurality of operating condition states includes a substantially fixed switch on time. A feedback time period signal generator generates a feedback time period signal received by the state machine. A period of the feedback time period signal is substantially greater than a period of the feedback sampling signal. The state machine is updated in response to the feedback time period signal.

REFERENCE TO PRIOR APPLICATION(S)

This is a continuation of U.S. application Ser. No. 12/703,108, filedFeb. 9, 2010, now pending. U.S. application Ser. No. 12/703,108 ishereby incorporated by reference.

BACKGROUND

1. Field of the Disclosure

The present invention relates generally to power supplies, and morespecifically, the invention relates to power supplies that have a slowloop bandwidth.

2. Background

Power supplies are typically used to convert alternating current (“ac”)power provided by an electrical outlet into direct current (“dc”) tosupply an electrical device or load. One important consideration forpower supply design is the shape and phase of the input current drawnfrom the ac power source relative to the ac input voltage waveform. Thevoltage waveform of mains ac sources is nominally a sinusoid. However,due to the non-linear loading that many switching power supplies presentto the ac source, the wave shape of the current drawn from the ac sourceby the power supply is non-sinusoidal and/or out of phase with the acsource voltage waveform. This leads to increased losses in the ac mainsdistribution system and, in many parts of the world, is now the subjectof legislative or voluntary requirements that power supply manufacturersensure the current drawn by the power supply is sinusoidal and in phasewith the ac voltage waveform.

The correction of the input current waveform in this way is referred toas power factor correction (PFC). If the input ac current and voltagewaveforms are sinusoidal and perfectly in phase, the power factor of thepower supply is 1. In other words, a power factor corrected input willpresent a load to the ac source that is equivalent to coupling avariable resistance across the ac source. The effective resistancepresented as a load to the ac source by the PFC corrected power supplyis varied as a function of the rms voltage of the ac source inaccordance with the power drawn by the PFC correct power supply outputload. As harmonic distortion and/or phase displacement of the inputcurrent relative to the ac source voltage increase, the power factordecreases below 1. Power factor requirements typically require powerfactors greater than 0.9 and may have requirements for the harmoniccontent of the input current waveform.

Applications where switching power supplies must provide PFC includeLight Emitting Diode (LED) lighting applications, which are becomingmore popular due to the improved energy efficiency provided by LEDscompared to more traditional incandescent lamps. Since the brightness oflight provided by LEDs is a function of the current flowing throughthem, the power supply also regulates the dc current provided to theLEDs, which form the output load to the power supply. The power supplycontrol therefore combines the functions of dc output current regulationand also provides PFC by presenting a substantially resistive load tothe mains ac source connected to the input of the power supply.

Output current regulation is typically achieved by sensing the currentflowing in the LEDs and providing a feedback signal that is a functionof the LED current to a power supply controller that regulates the flowof energy from an input to an output of the power supply. Switchingpower supplies will typically respond very quickly to fluctuations incurrent feedback information in order to regulate the LED current to bea smooth dc level.

As noted above, however, in order to achieve PFC, the power supply mustpresent a load that is essentially resistive to the ac mains. Rapidchanges in energy flow to regulate fast changes in LED current wouldcorrupt the PFC performance and yield non-sinusoidal power supply inputcurrent waveforms and low power factor. Therefore, in order to achievePFC, the power supply must be configured to respond slowly tofluctuations in current feedback information, which is often referred toas a slow power supply control loop or a low bandwidth loop. This slowloop functionality is normally achieved by introducing a largecapacitance within the power supply control loop. The capacitance mayfor example be introduced at the output of the power supply to maintaina very stable dc output voltage at the output of the power supply thatwill tend to reduce any current fluctuations in the LED load.

In another example, the current in the LED is allowed to fluctuate but alarge filter, typically comprising a large capacitance and resistance,is introduced in the feedback path between the LED current path and thepower supply controller. This then filters the feedback signal such thatthe power supply controller is responding to a heavily filtered versionof the power supply output current, which helps to prevent thecontroller making sudden demands for more or less energy flow from theac mains input source.

Both of the above-described techniques to achieve PFC have thedisadvantage of requiring physically large components in the powersupply to slow the power supply control loop response. Typicalapplications for LED lights require that the power supply circuitry beas compact as possible as they often have to fit inside very small lightbulb enclosures, sometimes referred to as in-bulb applications.Furthermore, large capacitors are a reliability and cost concern in suchin-bulb LED lighting applications since temperatures inside the bulb arehigh requiring the use of expensive high temperature capacitors.

BRIEF DESCRIPTION OF THE DRAWINGS

Non-limiting and non-exhaustive embodiments of the present invention aredescribed with reference to the following figures, wherein likereference numerals refer to like parts throughout the various viewsunless otherwise specified.

FIG. 1 illustrates an example schematic of a switch mode power converterincluding an example controller in accordance with the teachings of thepresent invention.

FIG. 2A is a functional block diagram illustrating an example controllerin accordance with the teachings of the present invention.

FIG. 2B illustrates a portion of a state machine state diagram for anexample controller in accordance with the teachings of the presentinvention.

FIG. 3 shows waveforms to illustrate the operation of an example powerconverter employing an example controller in accordance with theteachings of the present invention.

FIG. 4 is a functional block diagram illustrating an example controllerin accordance with the teachings of the present invention.

FIG. 5 shows waveforms to illustrate the operation of an example powerconverter employing an example controller in accordance with theteachings of the present invention.

DETAILED DESCRIPTION

In one aspect of the present invention, methods and apparatusesdisclosed here for explanation purposes use a power converter to providepower factor correction of an input current waveform. In the followingdescription, numerous specific details are set forth in order to providea thorough understanding of the present invention. It will be apparent,however, to one having ordinary skill in the art that the specificdetail need not be employed to practice the present invention.Well-known methods related to the implementation have not been describedin detail in order to avoid obscuring the present invention.

Reference throughout this specification to “one embodiment,” “anembodiment,” “one example” or “an example” means that a particularfeature, structure or characteristic described in connection with theembodiment is included in at least one embodiment or example of thepresent invention. Thus, the appearances of the phrases “in oneembodiment,” “in an embodiment,” “in one example” or “in an example” invarious places throughout this specification are not necessarily allreferring to the same embodiment. The particular features, structures orcharacteristics may be combined for example into any suitablecombinations and/or sub-combinations in one or more embodiments orexamples.

As will be discussed below, various examples in accordance with theteachings of the present invention allow a power factor corrected powerconverter controller to use a control technique that reduces the size,cost and number of external circuit components required to provide acompact power factor corrected power converter by providing a noveltechnique to dramatically slow the loop response of the power convertercontroller without the need for traditional external filteringtechniques. In this way an elegant, compact LED lamp converter can bemanufactured with the controller providing slow loop response with noadditional external components.

In one example the controller is coupled to drive a switch that isswitched on and off by the controller in a way to regulate the flow ofenergy from an input to an output of the power converter as will bedescribed in more detail below. In one example the controller samplesfeedback information representative of the output of the power converterat a feedback sampling frequency. In one example, the feedbackinformation representative of the output of the power converter isrepresentative of a current flowing in an LED load coupled to the outputof the power converter. In another example, the feedback information maybe representative of a voltage at the output of the power converter. Inone example, the feedback sampling frequency is substantially equal to aswitching frequency of the power switch. This feedback samplingfrequency is relatively high, meaning that, for example, more than 300such samples are taken during the period of one cycle of the ac mains.This period of the ac mains is typically 16 to 20 milliseconds(corresponding to 50 to 60 Hertz ac mains frequency) depending on wherein the world the circuit is operating.

In one example, the controller gathers feedback information in this wayfor a feedback time period, which in examples could be the period ofhalf or a whole ac mains cycle. In one example the controller sets asubstantially fixed operating condition of a power switch to becontrolled by the controller for the duration of the feedback timeperiod. As will be discussed in greater detail below, a fixed operatingcondition in this context could mean a substantially fixed switch ontime, a substantially fixed switching frequency, or the like, of theswitching of the power switch controlled by the controller to controlthe transfer of energy from an input of a power converter to an outputof the power converter.

In one example, the operating condition of the power switch ismaintained for the entire feedback time period and the next operatingcondition not set until the end of each feedback time period based onthe feedback information gathered at the feedback sampling frequencyduring that feedback time period. The feedback information is gatheredby counting the number of feedback samples above or below a firstfeedback threshold level during the feedback time period. In oneexample, the controller is configured to respond during the feedbacktime period if the feedback information representative of the output ofthe power converter exceeds a threshold level, which could indicate anabnormal condition that requires much more rapid response to provideprotection to the power converter or load for example.

To illustrate, FIG. 1 shows generally a schematic of an examplecontroller 109 included in a power supply, shown as power converter 100,in accordance with the teachings of the present invention. In theexample, power converter 100 is a flyback converter that is coupled to asource of ac voltage 101 at the input of the power converter 100.Typically, an ac voltage source is provided by an electricaldistribution system (e.g., power plant) through an electrical socket. Asshown in the example, a bridge rectifier 180 converts ac line voltage toa substantially unsmoothed dc input voltage waveform 104 of magnitudeVIN 106. In the illustrated example, capacitor 181 is of very low valueand is for the purpose of filtering high frequency noise currents andoffers substantially no smoothing of the rectified voltage waveform 104.

As illustrated in the example of FIG. 1, power converter 100 is shownincluding an energy transfer element, shown as a transformer 182, thatis coupled to a bridge rectifier 180 at one end and a power switch 116at an opposite end. In operation, power switch 116 is in an ‘on’ or‘closed’ state when power switch 116 is able to conduct current and inan ‘off’ or ‘open’ state when power switch 116 in unable to conductcurrent. In the example, an input return 183 is coupled to power switch116. In operation, current flows through energy transfer element 182when power switch 116 is on and current flows through energy transferelement 182 and output diode 184 for at least a portion of the time forwhich power switch 116 is off. In the illustrated example, therefore,the energy transfer element 182 transfers energy to an output of thepower converter 100 in response to the switching of power switch 116 inaccordance with the teachings of the present invention. In the example,therefore, power converter 100 is coupled to transfer energy from theinput terminals 115 to the load 111 coupled to output terminals 114. Inthe example, controller 109 drives power switch 116 on and off throughcoupling 118. In the example, power switch 116 and controller 109 formpart of integrated circuit 185, which could be manufactured as amonolithic (single die) or hybrid (multiple die) integrated circuit. Inother examples, the controller and power switch could be housed incompletely separate packages whilst still benefiting from the teachingsof the present invention.

As shown in the example, feedback signal 120 is coupled to controller109 through connection 117. In the example, the feedback signal 120 isgenerated across sense resistor 113, which provides a voltage VFBproportional to the current 119 flowing through load 111. In otherexamples, other current sense circuits such as current sensetransformers and the like could be used whilst still benefiting from theteachings of the present invention. In yet other examples, the powerconverter could be an isolated supply, in which case an opto-coupler, afeedback winding, a primary winding, or other way of isolating a signalrepresentative of the load current 119 and feedback signal 120 would beemployed whilst still benefiting from the teachings of the presentinvention.

In one example, controller 109 controls the switching of power switch116 to regulate a flow of energy from input terminals 115 to outputterminals 114 to provide an input current 102 having a waveform 105 thatis substantially in phase with and proportional to voltage waveform 104.In one example, controller 109 gathers feedback information fromfeedback signal 120 at a feedback sampling frequency generated internalto the controller 109. In one example, controller 109 gathers feedbackinformation from feedback signal 120 for a feedback time period, whichis substantially greater than a period of a feedback sampling signalused to sample the feedback signal. For instance, in one example, thefeedback time period is half of the ac mains period 103. In otherexamples, the feedback time period could be substantially equal to acomplete ac main cycle or another period of even longer duration. In oneexample, the controller 109 sets the operating condition of the switchcontrolled by the controller at the end of the feedback time period inresponse to the feedback information gathered during the feedback timeperiod.

In one example the feedback signal 120 is sampled substantially 320times for each feedback time period. In other words, in one example, theperiod of the feedback time period signal is at least 320 times longerthan the period of the feedback sampling signal, which is equal to areciprocal of the feedback sampling frequency. In another example, theperiod of the feedback time period signal is at least 500 times longerthan an average of the switching cycle periods during a feedback timeperiod. It is appreciated that although the feedback signal 120 is shownas a voltage signal, in other examples a feedback current signal couldbe used whilst still benefiting from the teachings of the presentinvention.

FIG. 2A shows an example block diagram of an integrated circuit 285comprising controller 209 and switch 216 benefiting from the teachingsof the present invention. In the example, controller 209 comprises afeedback sensor circuit 224, a state machine 222, a feedback time periodsignal generator 229 and a feedback sampling signal generator 221coupled together as shown. In the example, feedback terminal 210receives a feedback signal 220 that is a voltage relative to groundterminal 208 of integrated circuit 285. The feedback signal 220 iscompared to a threshold level Vref 230 at the input to comparator 225.In the example, comparator 224 is a threshold level sensing circuithaving a threshold level equal to the threshold level Vref 230. Theoutput of comparator 225 is coupled to counter 223, which is clocked byfeedback sampling signal 221. In the example, the output of counter 223is coupled to state machine 222. In one example, state machine 222 setsthe operating condition or state with which controller 209 will controlthe switching of switch 216 based on the value of the counter 223 at thetime when it is updated by feedback time period signal 226, which isgenerated by feedback time period signal generator 229. In one example,feedback sampling signal 227 has a period at least 320 times shorterthan the period of feedback time period signal 226. In other words,feedback sampling signal 227 clocks counter 223 at least 320 times moreoften than feedback time period signal 226 updates state machine 222 inthe example.

In one example, an operating condition or state set by state machine 222includes a fixed on time per switching cycle of switch 216 and/or afixed switching frequency for the switching of switch 216 at least untilthe next feedback time period signal 226 is received by state machine222. In other words, the switch 216 switching frequency and/or the ontime per switching cycle of switch 216 is unresponsive to the feedbacksignal 220 until at least such time that the feedback time period signal226 is received. After the next feedback time period signal 226 isreceived by state machine 222, depending on the value of the counter223, the state machine 222 then may then set the operating conditionstate to remain the same or the state machine 222 may set anotheroperating condition state to control the switching of the switch 216.

To illustrate, FIG. 2B shows one example of a portion of a statemachine, which in one example could be state machine 222 of FIG. 2A. Inthe example illustrated in FIG. 2B, three operating condition states270, 271 and 272 are shown with fixed switching frequency and fixedswitch on times. In another example, the switching frequency can bejittered to reduce electromagnetic interference (EMI). In one example,the full state machine state diagram could consist of 256 states. Inother examples, the full state machine state diagram could consist of agreater or fewer number of states than 256 states, depending on thegranularity desired for the state machine with respect to the particulardesign. In the example illustrated in FIG. 2B, in operating conditionstate (X) 270, the switching frequency is fixed at x kHz and the switchon time for every switching cycle is fixed at u μseconds.

With reference to FIG.2A, the value of the counter 223 could indicate tostate machine 222 that the next operating condition state is to changefrom operating condition state (X) 270 to operating condition state(X+1) 271 in FIG. 2B. Operating condition state (X+1) 271 has aswitching freq of y kHz, which is higher, and a switch on time of vμseconds, which is greater than the respective variables in operatingcondition state (X) 270. In the same way, in the example, operatingcondition state (X+2) 272 also has a switching freq that is higher and aswitch on time that is greater than those variables in operatingcondition state (X+1) 271. In one example, the change in switchingfrequency and switch on time per switching cycle could be selected suchthat the percentage change in power delivery by transitioning betweenoperating condition states is substantially constant. In one example,having the percentage change in power delivery between statessubstantially fixed helps to ensure that a power converter gain issubstantially fixed, independent of the particular states between whichthere is a transition. The way in which the transitions betweenoperating condition states are determined is described in more detailbelow with reference to FIG. 3.

FIG. 3 shows example waveforms that illustrate the operation of acontroller benefiting from the teachings of the present invention, whichin one example could be similar to controllers 109 and 209 in FIGS. 1and 2A, respectively. In the example, waveform 304 is a full-waverectified unsmoothed voltage waveform, which in one example correspondsto the waveform 104 in FIG. 1. In the example, waveform 305 is an inputcurrent waveform, which in one example corresponds to the waveform 105in FIG. 1. In the example, waveform 334 is a feedback signal waveform,which in one example corresponds to feedback waveforms 120 and 220 inFIGS. 1 and 2A, respectively, and is therefore representative of thecurrent flowing in the load.

It is noted that in the example of FIG. 3, output current waveform orfeedback waveform 334 is not in phase with the input current waveform305, which in one example could be due to the effects of an outputcapacitor coupled across the output of the power converter, such as forexample capacitor 112 in FIG. 1, which will tend to phase shift theoutput current waveform relative to the input current waveform. In oneexample, capacitor 112 of FIG. 1 helps to avoid excessive peak currentsin the load as will be discussed in more detail with respect to FIG. 5below.

It will be noted that despite being phase shifted from waveform 305,waveform 334 also has an overall period 332 that is substantiallyidentical to the cycle period 333 of the input voltage waveform 304. Inone example, therefore, this allows a feedback time period signal, suchas feedback time period signal 226 in FIG. 2A for example, to begenerated using any of waveforms 304, 305 or 334. If waveform 334 isused, the event of feedback signal waveform 334 transitioning from beinggreater than a feedback threshold value 330 to being less than thefeedback threshold value 330 can be used as the event to generate thefeedback time period signal 226. For example, time points 390 and 391 inFIG. 3 could be the start and end points of consecutive feedback timeperiods, with time point 390 indicating the start point of an n^(th)feedback time period and time point 391 indicating the end of the nthfeedback time period 392 and the start of the next feedback time period,which is shown as an (n+1)^(th) feedback time period 393. In otherwords, each of the periods of the feedback time period signal is in oneexample substantially equal to a time period between every second timethe feedback signal 334 crosses the threshold level 330. In anotherexample, the period of the feedback time period signal could instead besubstantially equal to time period 333, which is equal to one half cycleof the ac mains voltage waveform 304. In other words, in one example theperiod of the feedback time period signal is substantially equal to atime period between every zero voltage condition of a source of acvoltage coupled to the input of the power converter.

In one example, waveform 361 represents a counter output that isincremented or decremented at a rate of the feedback sampling signalperiod 369. In the example, if the feedback signal 334 is greater thanfeedback threshold level 330, the counter waveform 361 is incrementedevery feedback sampling period 369. Conversely, in the example, if thefeedback signal 334 is less than feedback threshold level 330, thecounter waveform 361 is decremented every feedback sampling signalperiod 369. In one example, the value of counter waveform 361 could besimilar to the output of counter 223 in FIG. 2A. In the example of FIG.3, the final value of the count represented by waveform 361 at timepoint 391, which is the end of nth feedback time period 392 in FIG. 3,is value 365. In one example, there is a hysteresis band defined bycount thresholds 363 and 364 as shown in FIG. 3.

With reference to the controller 209 in FIG. 2A, for example, if theoutput of the counter, shown as counter signal 249, has a value inbetween threshold count values 363 and 364, the state machine 222 setsan operating condition state for the next (n+1)^(th) feedback timeperiod that is unchanged from the operating condition state previouslyused for the n^(th) feedback time period. In other words, in oneexample, for the next operating condition state for the (n+1)^(th)feedback time period to be a different operating condition state, thefinal value of the count at time point 391 would have to be eithergreater than threshold level 363 or lower than threshold level 364.

It is appreciated that in other examples, multiple count thresholdsgreater than threshold 363 and less than threshold level 364 could beemployed. In one example, these multiple count thresholds could be usedto introduce a dynamic response determined in response to the value ofthe counter count 360 to determine the necessary change in the statemachine 222 operating condition state at the end of a feedback timeperiod. For example, if the final value of the counter count 360 at timepoint 391 was significantly higher the threshold 363, this couldindicate a significant reduction in a power supply loading, which couldin one example require a significantly lower state machine state. Byhaving multiple count thresholds above threshold 362, therefore, itwould be possible in one example to select a state machine 222 operatingcondition state appropriate to the magnitude of the counter count 360.

This operation can be further illustrated with reference to FIG. 2Babove. In an example where the controller is operating in operatingcondition state (X) 270 during n^(th) feedback time period 392, it willremain in operating condition state (X) 270 for the next (n+1)^(th)feedback time period 393 in the example shown where final count value365 is between hysteresis threshold count levels 363 and 364. In anotherexample, however, if the final count value were less than thresholdcount level 364, then this would indicate that over the precedingfeedback time period, the feedback signal, for example feedback signal220, was below the feedback threshold level Vref 230, for example, morethan it was above the feedback threshold level Vref 230 during thatfeedback time period. In this example, it would then be necessary toincrease the power delivered to the load, for example load 111 inFIG. 1. In one example therefore, under such conditions, the statemachine 222 would transition from operating condition state (X) 270 tooperating condition state 271 (X+1).

As described with reference to FIG. 2A above, in other examples,multiple counter thresholds could be employed in addition to thresholdcount levels 363 and 364. In such examples, the state machine 222 couldfor example transition from state (X) 270 to state (X+2) if the countvalue at the end of a feedback time period indicated that a moresignificant increase in power delivery to the load, for example load 111in FIG. 1, were necessary.

In the example of FIG. 3 the count value is always reset at the end ofeach feedback time period to the threshold count level 362. In theexample of FIG. 2A, this reset of the counter 223 is represented by theapplication of feedback time period signal 226 to the RESET input ofcounter 223. This reset ensures that each feedback time period respondsto the feedback information gathered during that feedback time periodand therefore does not accumulate errors from one or more precedingfeedback time periods that could lead to power converter instability.

FIG. 4 shows an example block diagram of an integrated circuit 485comprising a controller 409 and a switch 416 benefiting from theteachings of the present invention. In one example, the operation ofcontroller 409 shares many aspects with controller 209 in FIG. 2A. Forinstance, in the example, controller 409 comprises a feedback sensorcircuit 424, a state machine 422, a feedback time period signalgenerator 429 and a feedback sampling signal generator 421 coupledtogether as shown. In the example, feedback terminal 410 receives afeedback signal 420 that is a voltage relative to ground terminal 408 ofintegrated circuit 485. The feedback signal 420 is compared to athreshold level Vref 430 at the input to comparator 425. The output ofcomparator 425 is coupled to counter 423, which is clocked by feedbacksampling signal generator 421. In the example, the output of counter 423is coupled to state machine 422. In one example, state machine 422 setsthe operating condition or state with which controller 409 will controlthe switching of switch 416 based on the value of the counter 423 at thetime when it is updated by feedback time period signal 426 that isgenerated by feedback time period signal generator 429. In one example,feedback sampling signal 427 has a period at least 320 times shorterthan the period of feedback time period signal 426. In other words,feedback sampling signal 427 clocks counter 423 at least 320 times moreoften than feedback time period signal 426 updates state machine 422 inthe example.

FIG. 4 also shows that feedback time period signal generator 429includes an input 471, which in one example may be coupled to the inputor output of the power converter to obtain timing information used togenerate the feedback time period signal 426. In one example, the periodof the feedback time period signal 426 is substantially equal to a timeperiod between every zero voltage condition of a source of ac voltagecoupled to an input of the power converter, such as for example timeperiod 333 as illustrated in FIG. 3. In another example, feedback timeperiod signal 426 is substantially equal to a time period between everyother time where the feedback signal 334 crosses the feedback threshold330, such as for example period 332 as illustrated in FIG. 3.

FIG. 4 also shows feedback sampling signal generator 421 coupled toreceive an input 472, which in one example could be the feedback timeperiod signal 426 from feedback time period signal generator 429. In oneexample, feedback time period signal 426 is used by feedback samplingsignal generator 421 to adjust the period of feedback sampling signal427 to ensure that a substantially fixed number of feedback samplingsignals 427 are provided for every single feedback time period signal426. In one example, this function is similar to a phase locked loopcircuit that will be familiar to ones skilled in the art. In oneexample, the period of feedback sampling signal 427 is adjusted toensure that 320 feedback sampling signals 427 are provided for everysingle feedback time period signal 426. In other examples, a greater orfewer number of feedback sampling signals 427 than 320 may be includedin every feedback time period signal 426. The tradeoffs for having agreater number of feedback signals would be the benefit of having higherresolution and sensitivity in exchange for the increased cost andcomplexity of the circuitry to support the increased number of feedbacksampling signals 427.

The example illustrated in FIG. 4 also shows a second threshold levelsensing circuit, illustrated as second comparator 442, which is includedwithin feedback sensor circuit 424, with one input coupled to receivefeedback signal 420 and the other input coupled to receive a secondfeedback threshold Vref2 431, which in one example is greater than Vref430. In the example, the output signal 443 from comparator 442 iscoupled to enable/disable state machine 422 from switching switch 416 inresponse to a comparison of feedback signal 420 with Vref2 431. In oneexample, signal 443 transitions from high to low due to feedback signal420 exceeding Vref2 431. In operation, logic 440, which is illustratedas an AND gate in FIG. 4, is coupled to receive signal 443 toenable/disable state machine 422 as shown. As shown in the example,logic 440 disables the switch 416 until such time that the feedbacksignal no longer exceeds Vref2 431. In one example, this operationprovides protection for the power converter in which controller 409 isused and also protects the output load coupled to the power converter inwhich controller 409 is used as will be described below.

FIG. 5 shows feedback waveforms that illustrate the effect of using avery small value output capacitor, such as for example capacitor 112 inFIG. 1, with and without the additional circuitry described above withreference to FIG. 4. As will be discussed, waveform 535 can result ifcomparator 442 and the associated circuitry described above, such as forexample controller 209 in FIG. 2A, are not used. Waveform 536illustrates an example of a feedback waveform with a controllerbenefiting from the improvements of FIG. 4

With regard to waveform 535, when compared to the more symmetricalfeedback waveform 334 in FIG. 3, waveform 535 has much higher peakvalues corresponding to high peaks in load current during the feedbacktime period 532. In one example, such high peak currents can be damagingto both the load, such as for example the LED load 111 in FIG. 1, andalso power converter output components, such as for example output diode184 in FIG. 1. In one example a feedback waveform such as waveform 535could reduce the lifetime of the power converter and load, which istypically not acceptable. However, it is often attractive to reduce thevalue of the output capacitor as much as possible for space and costsaving reasons. The introduction of the improvements described in theexample of FIG. 4 above therefore allow the most positive peaks of thewaveform 535 to be limited to a second threshold feedback level 531,which in one example could be similar to second feedback threshold levelVref2 431 in FIG. 4.

It is noted that because the example controller 409 in FIG. 4 willrespond with substantially no delay to feedback signal values exceedingsecond threshold level 431, the power factor of the power converter willbe degraded to some extent. However, since power factor targets fortypical applications such as LED lighting are in the range of 0.7 to0.9, some degradation in power factor is still acceptable and that thecost and space savings of using a small output capacitor can make thisan attractive trade off.

In addition, it is noted that the capacitance value of outputcapacitors, such as for example capacitor 112 in FIG. 1, may decreaseover time during the lifetime of a power converter. As such, in oneexample, the additional circuitry described in FIG. 4 can protect theload and power converter as the capacitance of the output capacitordecreases over time during the lifetime of the power converter.

The above description of illustrated examples of the present invention,including what is described in the Abstract, are not intended to beexhaustive or to be limitation to the precise forms disclosed. Whilespecific embodiments of, and examples for, the invention are describedherein for illustrative purposes, various equivalent modifications arepossible without departing from the broader spirit and scope of thepresent invention.

These modifications can be made to examples of the invention in light ofthe above detailed description. The terms used in the following claimsshould not be construed to limit the invention to the specificembodiments disclosed in the specification and the claims. Rather, thescope is to be determined entirely by the following claims, which are tobe construed in accordance with established doctrines of claiminterpretation. The present specification and figures are accordingly tobe regarded as illustrative rather than restrictive.

1. A controller for use in a power converter connected to an ac input,comprising: a feedback sensor circuit coupled to receive a feedbacksignal representative of an output of the power converter; a feedbacksampling signal generator coupled to generate a feedback sampling signalcoupled to be received by the feedback sensor circuit, wherein thefeedback sensor circuit is coupled to sample the feedback signal inresponse to the feedback sampling signal; a state machine coupled to thefeedback sensor circuit to control switching of a switch of the powerconverter circuit according to one of a plurality of operating conditionstates in response to the feedback sensor circuit, wherein each of theplurality of operating condition states includes a substantially fixedswitch on time; and a feedback time period signal generator coupled togenerate a feedback time period signal coupled to be received by thestate machine, wherein a period of the feedback time period signal issubstantially greater than a period of the feedback sampling signal,wherein the state machine is coupled to be updated in response to thefeedback time period signal.
 2. The controller of claim 1 wherein afrequency of the feedback sampling signal is substantially equal to aswitching frequency of the switch.
 3. The controller of claim 1 whereinthe switching of the switch is coupled to control a transfer of energyfrom the ac input of the power converter to the output of the powerconverter.
 4. The controller of claim 3 wherein the switching of theswitch is coupled to control the transfer of energy such that an inputcurrent flowing in the ac input of the power converter is substantiallyin phase with and proportional to an ac voltage at the ac input of thepower converter.
 5. The controller of claim 1 wherein the feedbacksignal representative of the output of the power converter isrepresentative of a current flowing through the output of the powerconverter.
 6. The controller of claim 1 wherein the feedback signalrepresentative of the output of the power converter is representative ofa voltage at the output of the power converter.
 7. The controller ofclaim 1 wherein each of the plurality of operating condition statesincludes a substantially fixed switching frequency and the substantiallyfixed switch on time.
 8. The controller of claim 1 wherein the period ofthe feedback time period signal is at least 320 times longer than theperiod of the feedback sampling signal.
 9. The controller of claim 1wherein each period of the feedback time period signal is substantiallyequal to a time period between every zero voltage condition of a sourceof ac voltage coupled to the ac input of the power converter.
 10. Thecontroller of claim 1 wherein the feedback sensor circuit includes acounter coupled to a first threshold level sensor circuit having a firstthreshold level coupled to receive the feedback signal.
 11. Thecontroller of claim 10 wherein each of the periods of the feedback timeperiod signal is substantially equal to a time period between everysecond time the feedback signal crosses the first threshold level. 12.The controller of claim 10 wherein the counter is coupled to be clockedin response to the feedback sampling signal at a feedback samplingfrequency.
 13. The controller of claim 10 wherein the counter is coupledto be incremented by the first threshold level sensor circuit inresponse to an output of a comparison of the feedback signal with thefirst threshold level being in a first state.
 14. The controller ofclaim 13 wherein the counter is coupled to be decremented by the firstthreshold level sensor circuit in response to an output of a comparisonof the feedback signal with the first threshold level being in a secondstate.
 15. The controller of claim 10 wherein the state machine iscoupled to set an operating condition state in response to an outputvalue of the counter to control switching of the switch of the powerconverter.
 16. The controller of claim 15 wherein the state machine iscoupled to set a next operating condition state at an end of each periodof the feedback time period signal in response to the output value ofthe counter.
 17. The controller of claim 15 wherein the state machine iscoupled to maintain an operating condition state until an end of eachperiod of the feedback time period signal.
 18. The controller of claim10 wherein the counter is coupled to be reset at an end of each periodof the feedback time period signal.
 19. The controller of claim 10wherein the feedback sensor circuit further includes a second thresholdlevel sensor circuit having a second threshold level coupled to receivethe feedback signal.
 20. The controller of claim 19 further comprisinglogic coupled to the state machine to disable the state machine fromswitching the switch of the power converter circuit in response to acomparison of the feedback signal with the second threshold level.